The present invention claims priority to Japanese Application No. P2000-210326 filed Jul. 11, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell for use as a power source of small and light-weight electric apparatuses and electric automobiles.
2. Description of the Related Art
Recently, various portable electronic apparatuses such as a video camera with video tape recorder, cellular phones, and lap-top computers have been developed and these electronic apparatuses are expected to be reduced in size and weight. Simultaneously with this, various studies are made to improve an energy density of secondary cells used as a drive power source of these electronic apparatuses.
Particularly, a great expectation is paid on non-aqueous electrolyte secondary cells using a lithium metal, a lithium alloy or a material capable of doping/dedoping lithium as an anode material and a composite oxide as a cathode material, i.e. so-called lithium-based secondary cells which have a higher energy density and a smaller weight as compared to conventional lead-acid cells or nickel-cadmium secondary cells.
For example, as a cathode active material, a non-aqueous electrolyte secondary cell using LixCoyO2 having a high discharge potential and a high energy density, i.e., lithium-ion secondary cells are now used in practice.
Moreover, for electric automobiles or load leveling, large-size non-aqueous electrolyte secondary cells are now developed in various fields. As the cell size is increased, more cathode active material is required. Thus, a great amount of raw materials of the cathode active material is required.
However, cobalt as a raw material of LixCoyO2 is rare in resources and cobalt deposits commercially usable are present in few countries, which makes cobalt expensive and its price fluctuates much. There is no guarantee to assure to obtain cobalt in future.
Accordingly, in order to provide such non-aqueous electrolyte secondary cells, it is desirable to obtain a cathode active material using raw materials rich in resources and having the performance equivalent to or more than LixCoyO2.
As such a cathode active material, there has been suggested to use LixNiO2 or LixMn2O4 (the value of x depends on discharge/charge and is normally x and y are approximately 1 during synthesis) having a high discharge potential and a high energy density equivalent to LixCoyO2. These raw materials, i.e., nickel and manganese are cheap as compared to cobalt. Especially manganese is cheaper than nickel and rich as a resource. Moreover, manganese dioxide is widely used as a material of manganese dry cells, alkaline-manganese dry cells, and lithium primary cells. This assures to supply manganese dioxide without fail. Thus, a study of a cathode active material using manganese is widely made.
There have been suggested various composite oxides using lithium and manganese (hereinafter, referred to as a lithium-manganese composite oxide) synthesized from various manganese raw materials and lithium raw materials such as LixMnyO4(x≈1,y≈2) having the spinel crystal structure.
The lithium-manganese composite oxide expressed as LixMnyO4(x≈1,y≈2) is electrochemically oxidized so as to show potential of 3V or more for lithium and has a theoretical charge/discharge capacity of 148 mAh/g.
Moreover, as the electrolyte of such non-aqueous electrolyte secondary cells when using a non-aqueous electrolyte solution prepared by an electrolyte salt solved in a non-aqueous solvent, what is used most widely is the one using LiPF6 as an electrolyte salt solved in a carbonate-based non-aqueous solvent such as propylene carbonate and diethyl carbonate which has a high electrical conductivity and a stable potential.
However, when using manganese oxide or lithium-manganese composite oxide used as the cathode active material, there arises a problem of deterioration of the cell characteristic due to the charge/discharge cycle.
It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary cell using as a cathode active material a manganese oxide that can be prepared at a low cost or a lithium-manganese composite oxide which prevent deterioration of the cell characteristic even after repeatedly used.
In order to achieve the aforementioned object, the present invention provides a non-aqueous electrolyte secondary cell having: a cathode containing a manganese oxide or a lithium-manganese composite oxide; an anode containing a lithium metal, a lithium alloy, or a material capable of doping/dedoping lithium; and an electrolyte containing at least two electrolyte salts, which electrolyte contains LiBF4 as one of the electrolyte salt in a range from 0.005 mol/l to 0.3 mol/l.
Because the non-aqueous electrolyte secondary cell having the aforementioned configuration according to the present invention contains LiBF4 as one of the electrolyte salts, a stable film is formed on the anode surface.